OS 10.4 - DVD reading Problem

Guys give me some idea
Mine is iMac G4 , 10.4.9 and my computer does not read any DVDs not even operating system DVD
if i insert blank DVD then it shows as blank dvd but not movies or anything else not even the dvd which i Write in my computer
i want to boot and reinstall Mac osx10.4
What do i do?
Please help

i tried that one as well
On System Pref> System> start up disk > it only shows "Mac OsX 10.4.9 on 3" and "Network start up"
On disk utilities It doesnt show Disk on left panel
on Finder> Pref>it is checked on cd,dvd,ipod